linux-kernel - Re: [PATCH] cgroup: pids: use {READ,WRITE}

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Message-ID: <20191016153520.zet5mn5xsygig4xc@yavin.dot.cyphar.com>
Date:   Thu, 17 Oct 2019 02:35:20 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit
 operations

On 2019-10-17, Aleksa Sarai <cyphar@...har.com> wrote:
> On 2019-10-16, Tejun Heo <tj@...nel.org> wrote:
> > Hello, Aleksa.
> > 
> > On Wed, Oct 16, 2019 at 07:32:19PM +1100, Aleksa Sarai wrote:
> > > Maybe I'm misunderstanding Documentation/atomic_t.txt, but it looks to
> > > me like it's explicitly saying that I shouldn't use atomic64_t if I'm
> > > just using it for fetching and assignment.
> > 
> > Hah, where is it saying that?
> 
> Isn't that what this says:
> 
> > Therefore, if you find yourself only using the Non-RMW operations of
> > atomic_t, you do not in fact need atomic_t at all and are doing it
> > wrong.
> 
> Doesn't using just atomic64_read() and atomic64_set() fall under "only
> using the non-RMW operations of atomic_t"? But yes, I agree that any
> locking is overkill.
> 
> > > As for 64-bit on 32-bit machines -- that is a separate issue, but from
> > > [1] it seems to me like there are more problems that *_ONCE() fixes than
> > > just split reads and writes.
> > 
> > Your explanations are too wishy washy.  If you wanna fix it, please do
> > it correctly.  R/W ONCE isn't the right solution here.
> 
> Sure, I will switch it to use atomic64_read() and atomic64_set() instead
> if that's what you'd prefer. Though I will mention that on quite a few
> architectures atomic64_read() is defined as:
> 
>   #define atomic64_read(v)        READ_ONCE((v)->counter)

Though I guess that's because on those architectures it turns out that
READ_ONCE is properly atomic?

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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